The present invention generally relates to a transmitter and transmission control method in radio communication.
The OFDM (Orthogonal Frequency Division Multiplexing) transmission system is a promising system since it makes it possible to receive radio signals without interference between codes even under a multi-path environment. A transmission structure employed in the OFDM transmission system is explained with reference to FIG. 1.
A transmitter 1 comprises a symbol generator 2 receiving information bits, a serial-to-parallel (S/P) converter 3 connected to the symbol generator 2, an inverse fast Fourier transformer (IFFT) 4 connected to the S/P converter 3, a parallel-to-serial (P/S) converter 5, a GI adder 6 connected to the P/S converter 5 and an antenna 7 connected to the GI adder.
The information bits are input to the symbol generator 2. Similar to single carrier transmission, the symbol generator 2 performs error correction encoding, interleaving, symbol mapping, etc., on the received information bits to generate transmission symbols, and provides the generated transmission symbols to the S/P converter 3. The S/P converter 3 performs serial-to-parallel conversion of the received transmission symbols, and provides the parallel signals to the IFFT 4. The IFFT 4 converts the input signals to orthogonal multi-carrier signals, and provides the resultant signals to the P/S converter 5. The P/S converter 5 performs parallel-to-serial conversion of the received signals, and provides the serialized signals to the GI adder 6. The GI adder 6 makes a partial copy of the received signals and adds the partial copy to the received signals as a guard interval. The signals accompanied by the guard interval are transmitted via the antenna 7.
The above mentioned OFDM transmission system has a PAPR (Peak to Average Power Ratio) problem, in which signals whose amplitudes are significantly large compared with average amplitude appear in the OFDM modulated signals, which are output signals of the IFFT 4.
This problem is unique to multi-carrier modulation systems. When separately modulated, many carriers are synthesized in the same phase, and the added outputs may become very large and have a peak that is very high compared with the average output. The maximum peak power can be a value of the average power multiplied by the maximum number of used sub-carriers.
A transmission amplifier has a limited linear region where its input and output are linear. When a signal exceeding the linear region is input to the amplifier, an output waveform becomes distorted, resulting in problems such as transmission quality degradation, power radiation to outside of the bandwidth, etc. On the other hand, if the linear region is widened, amplification efficiency becomes lower. Therefore, the amplitude (power) distribution of transmission signals is desired to be rather flat without a high peak. In order to reduce PAPR, a partial transmit sequence (PTS) method and a cyclic shift method are proposed.
Next, a transmitter performing peak reduction is explained with reference to FIG. 2.
The transmitter 1 shown in FIG. 2 has a low peak IFFT unit 8 to which the PTS method or the cyclic shift method is applied, instead of the IFFT 4 used in the above explained transmitter shown in FIG. 1 (See Non-Patent Document #1).
The structure of the low peak IFFT unit 8 is explained below with reference to FIG. 3, in which the input signals are divided or partitioned into two groups for inverse fast Fourier transform.
The low peak IFFT unit 8 comprises partitioned IFFT 8-1, and a peak reducer 8-2 connected to the partitioned IFFT 8-1. The peak reducer 8-2 comprises peak reduction processors 8-21 and 8-22 connected to the partitioned IFFT 8-1, a peak reduction controller 8-23 connected to the partitioned IFFT 8-1 and the peak reduction processors 8-21 and 8-22, and an adder unit 8-24 connected to the peak reduction processors 8-21 and 8-22. The adder unit 8-24 is equipped with plural adders connected to the peak reduction processors 8-21 and 8-22. An output signal from the adder unit 8-24 is input to the P/S converter 5.
In this structure, the partitioned IFFT 8-1 receives plural sub-carriers, divides them into plural groups, for example, NG groups (NG is an integer larger than 2), and perform inverse fast Fourier transform (IFFT) on the sub-carriers in groups.
The structure of the partitioned IFFT 8-1 is explained below with reference to FIG. 4, in which 8 point inverse fast Fourier transform is performed divided into two parts.
The partitioned IFFT 8-1 comprises a first IFFT 8-11 and a second IFFT 8-12. When generating time domain signals corresponding to f(0)˜f(3), for example, signals on which inverse fast Fourier transform should be performed are input to terminals f(0)˜f(3) of the first IFFT 8-11, and zeros are input to terminal f(4)˜f(7). In this structure, two IFFTs are used and therefore twice the amount of calculation is needed, compared with no partitioned structure.
The time domain signals output from the first IFFT 8-11 correspond to the sub-carriers f(0)˜f(3). The time domain signals output from the second IFFT 8-12 correspond to the sub-carriers f(4)˜f(7). In normal IFFTs, the signals from the first and second IFFTs and having the same timing point are added.
In this PAPR method using the cyclic shift method and the PTS method, the peak reduction processors 8-21 and 8-22 perform cyclic shift or phase rotation on these signals [F(0), F(1), . . . F(NFFT-1)], and thereafter the adder unit 8-24 adds them together and outputs the results. The NFFT (NFFT is an integer larger than 1) means the total number of the Fourier transform points. The peak reduction controller 8-23 controls the amount of the cyclic shift or phase rotation so as to lower peaks in the output signals. In this manner, high peak occurrence is suppressed.
When employing the PTS method or the cyclic shift method, the amount of the phase rotation or the cyclic shift should be sent to a receiver as control information. In order to do this, a control signal can be used as disclosed in Non-Patent Documents #3, #4.
[Non-Patent Document #1]
L. J and N. R. Sollenberger, “Peak-to-Average power ratio reduction of an OFDM signal using partial transmit sequences”, IEEE Commun. Lett., vol. 4, no. 3, pp. 86-88, March 2000.
[Non-Patent Document #2]
Seog Geun Kang, et. al. “A Novel subblock partition scheme for partial transmit sequence OFDM”, IEEE Trans. on Broadcasting vol. 45, no. 3, pp. 333-338. Sep. 1999
[Non-Patent Document #3]
Wong Sai Ho et. al., “Peak-to-average power reduction using partial transmit sequence: a suboptimal approach based on dual layered phase sequencing”, IEEE Trans. on Broadcasting, vol. 49, no. 2, pp. 225-231, June 2003
[Non-Patent Document #4]
A. D. S. Jayalath et. al., “Reduced complexity PTS and new phase sequences for SLM to reduce PAP of an OFDM signal”, Proc. of IEEE VTC 2000, vol. 3, pp. 1914-1917.
However, the above explained related art examples have the following problems.
When plural sub-carriers are divided into plural groups and receive IFFT processing, the calculation amount required for the IFFT is increased by a multiple of the number of divided groups.
The peak reduction control becomes complicated depending on the number of input symbols.